1. Field of the Invention
The invention relates to a method of coating the dielectric filaments with dielectric coatings based on plasma chemical vapor deposition (CVD) mechanism. The method is especially useful for coating optical fibers. The device for realization of the proposed method is also described.
2. Information Disclosure Statement
Numerous methods and devices have been proposed to coat optical fibers with dielectric hermetic coatings by plasma chemical vapor deposition. Plasma CVD methods in general show certain advantages over conventional chemical CVD methods. For instance, it is possible to obtain diamondlike carbon layers or diamond layers on substrates only with plasma CVD with high deposition rates and at reasonable pressures. Such layers are, for instance, desired as a hermetic, abrasion resistant protection for optical quartz glass fibers. Such diamondlike coatings are substantially harder--and therefore more durable than the graphitic layers being deposited by conventional CVD as described, for instance, in U.S. Pat. No. 5,000,541. U.S. Pat. No. 5,178,743 describes a cylindrical magnetron sputtering system. While this system may be well applicable to deposit various films on filaments, the magnetic confinement mechanism used therein can only be effective at very low pressures (several milliTorr to maximum 100 milliTorr). This creates sealing problems at the filament inlet and outlet to the deposition chamber. Furthermore the deposition length of such a device is practically very limited. Other potential methods include long electrodes being placed around the moving filament to generate a HF plasma or a microwave resonator around a tubular deposition chamber, generating a cylindrical plasma zone, through which the fiber is passing.
In all these cases the plasma volume is considerably larger than the volume occupied by the filament to be coated. While this is a general problem encountered with the coating of any filaments by plasma. CVD, the geometry of optical fibers (diameters typically 125 .mu.m to 1 mm) is particularly mismatched with the large plasma volumes generated in order to coat their small surfaces. This results in a majority of the deposition taking place not on the filament--as desired--but on the electrodes, the chamber walls and other secondary surfaces. This deposition can crack of during the coating runs, create dust and thus result in film deficiencies on the filament itself. The plasma distribution can be non-uniform, high gas consumption and large amounts of residual gas are further consequences of the unnecessarily large plasma volumes. Thus, it would be desirable to generate the plasma only in the close vicinity of the filament, where it contributes to the deposition process.